1. Technical Field of the Invention
The present invention pertains to a sulphur free prevulcanized rubber latex, and to a method of producing same and articles produced therefrom. More particularly, the present invention pertains to a method of manufacturing sulfur-free, natural rubber latex and for forming same into thin walled articles such as gloves so that the resulting articles are not chemically reactive with other substances which may be contacted by the articles during subsequent use, and so that the articles are otherwise completely safe for substantially any application, including those involving with humans.
2. Description of Relevant Art
Rubber is a very common compound. It is available in many different compositions and forms such as rubber latex. In conventional processes for forming rubber, or for forming products from rubber, rubber is cured or vulcanized so as to join the modules within the rubber composition together giving the rubber composition desired strength and toughness. Most typically sulphur or sulphur donating compounds are used as the primary curing or vulcanizing agents.
For example, there are many publications available, including publications by Uniroyal, Vanderbilt and Monsanto describing start-formulas or for sulphur-cure natural rubber systems. Typically, these formulas will include sulphur or a partial substitute of sulphur such as TMTD, DIAK and Sulfasan (partial substitutes of sulfur being preferred especially when controlled release of available sulphur is required), a zinc-based cross linking agent such as zinc dithiocarbamate (ZDBC) and zinc diethylcarbamate (ZDEC), cure rate accelerators (primary and secondary) such as excessive sulphur, ZDBC and sodium diethylcarbamate (SDBC), and an antioxidant which are mostly phenolic or amine compounds.
Although products formed from such sulphur-cured natural rubber exhibits some very good qualities, including strength, toughness, elasticity, etc. and are used very effectively in many applications, there are some areas of applications where compatibility problems arise due to the chemicals incorporated into the rubber which react undesirably with foreign materials contacted by products formed from the rubber during the products""subsequent use. Particularly, the sulphur, sulphur compounds, dithiocarbamate compounds, zinc and its compounds which are present on and/or in the surfaces of products formed from the rubber may react with foreign materials contacted during use of the products, resulting in contamination, fouling, spoilage or discoloration of rubber products or of the materials contacted therewith (either directly or indirectly). Additionally, nitrosamine, which is often used as an antioxidant in conventional rubber compositions or is generated by the amine compounds used as antioxidants, is believed to be carcinogenic, and hence it is undesirable to use these compounds as antioxidants where products formed from the rubber composition are likely to be contacted by humans, such as with their skin, body fluids, or in their mouths.
A specific example of a problem which has arisen due to a chemical reaction between chemicals in a conventional rubber product and a foreign material contacted by the rubber product during use is in the field of latex examination gloves. Such gloves are almost always used by persons in the medical and dental field during any procedure requiring bodily contact with a patient, such use is even required by federal and/or state laws in many instances and such gloves are also being used in many other fields. Throughout the application the term xe2x80x9clatex glovesxe2x80x9d refers to any type of latex glove including examination gloves, surgical gloves, and all purpose gloves.
A common procedure involved in dental examinations is the taking of an impression of a patient""s upper or lower dental arch, or of one or several teeth. Such impressions are formed using a gel-like dental impression material (DIM) in a metal or plastic container which is pressed against the dental structure to obtain a negative impression of the patient""s teeth or arch. Upon removal of the impression material and container from the patient""s oral cavity, it is allowed to cure and harden. Once hardened, a second material such as a flowable plaster is poured into the hardened, negative impression material to obtain a positive impression of the teeth or arch. One such DIM is vinyl polysiloxane distributed by 3M under the Trademark EXPRESS(trademark).
The dental community has recently observed a re-occurring problem in forming negative impressions from the DIM caused by chemical reaction(s) with chemicals contained on/in conventional rubber latex gloves. Specifically, after a negative impression has been obtained of the patient""s dental structure, portions of the exposed regions of the DIM which had been in contact (direct or indirect) with the gloves fail to sufficiently cure and harden. Instead, those regions remain uncured and retain their gel-like consistency. This is most undesirable when attempting to subsequently form the positive impression since the uncured regions of the negative impression do not sufficiently retain and transfer the structure and surface details of the patient""s teeth or arch which had been previously contacted, to the positive impression material. Regions of the DIM remain uncured because chemicals on/in the latex gloves react with and tie up the platinum base catalyst of the DIM, which catalyst normally assures rapid curing and hardening of the DIM.
Previously, applicant has proposed to overcome such glove contamination problem through post treatment of the manufactured gloves to tie up and/orbarricade the offending chemicals on/inthe gloves. Particularly, applicant has proposed to tie up the offending chemicals by treating the exterior surfaces of the latex glove with an acidic suspension of metal particles, such as relatively dilute solution of phosphoric acid (0.25-2.0 molar) with finally divided copper and/or iron particles therein. The suspension is applied to the glove exterior in any appropriate manner such as spraying, brushing, wiping, dipping, etc. after which the treated gloves are allowed to dry. Alternatively, or additionally, a water resistant (preferably insoluble) barricade coating was applied to the exterior of the gloves, including such coating materials as silicone spray, polyethylene, PVC, polysilane, octyl acrylamid, acrylate copolymer VA, chrononates, vinyl neodecanoate copolymer, ethylester of polyvinyl, nathacrylate-methylacrylate copolymer, etc.
Although such post treatments of the conventional latex gloves were usually effective for overcoming undesirable chemical reactions between chemicals of the latex gloves and foreign materials such as the DIM contacted by the latex gloves during use, the treated gloves still undesirably reacted with the DIM approximately 10% of the time and this was an unacceptable failure rate. Further, such post treatments have other problems and disadvantages associated therewith, including the additional cost involved with the post treatments, shelf life and reliability of the post treated gloves, undesired reactions of the post treatment materials with other foreign materials subsequently contacted by the gloves, etc.
The curing and vulcanization of rubber compounds may be done at various stages relative to the formation or shaping of products from the rubber compounds. For example, in the manufacture of latex gloves from latex emulsions, it is conventional to vulcanize the composition after it has been shaped into gloves (post-vulcanization process), or the latex emulsion may be vulcanized before it is shaped into the gloves (pre-vulcanization). Typically, the chemicals used for vulcanizing the rubber compositions are consumed more efficiently in pre-vulcanization processes because vulcanization occurs with respect to a large mass of material over a longer length of time. On the other hand, pre-vulcanized gloves are known to be tacky and soft in comparison to the post-vulcanized gloves, which leads to handling problems.
Another problem associated with rubber latex gloves pertains to the latex proteins inherently found therein. Latex proteins, which exist naturally in natural rubber latex ,are essential an emulsifier to the polyisoprene and thus maintain the latex state. Unfortunately, the proteins have caused allergic reactions and other serious health problems in humans, and the latex proteins appear to have a relative progressive effect on humans such that the undesirable reactions increase in severity with increased exposure to the proteins. Conventionally, latex gloves include a powder such as corn starch on the surfaces thereof which facilitates removal of the gloves from a mold and facilitates placing gloves on and removing the gloves from users""hands. Although the powder acts as a layer between the latex glove and the hand wearing the glove, the latex proteins readily pass through the powder to the skin. In light of the health problems associated with exposure to the latex proteins, two alternative treatments for the gloves have been conventionally utilized to significantly reduce or eliminate the possibility that persons wearing the latex gloves will contact the latex proteins therein. Gloves treated according to such additional treatments are known as xe2x80x9cpowder-freexe2x80x9d. The first of such additional treatments is chlorination of the gloves, which removes some of the proteins on the gloves. Such chlorination treatment actually weakens the gloves because it initiates a deterioration process, but conventional sulfur-cured gloves remain sufficiently strong even after chlorination. The second of such additional processes is a wax coating process whereby the surfaces of the conventional powdered gloves are coated with an ingestible, biodegradable wax material. The wax material may be carnauba wax, which is the same as that used on the candy shells of certain candy products such as MandMs(copyright). Although such wax coating does not degrade the desired characteristics of the latex glove, it does tend to melt to some extent after being maintained at body temperature over a period of time, and this is undesirable because the melted wax leaves a residue on users""hands after the gloves are removed, which residue is often subsequently transferred to instruments or other articles handled by the users.
The present invention has been developed to overcome the problems and disadvantages of known rubber latex gloves as discussed above, and to fulfill a great need in the art by providing a composition which is essentially free of active chemical compounds (such as sulphur, diocarbamates, zinc compounds, etc.) found in conventional sulphur-cured rubber latex products, which is free of other or undesirable chemicals such as nitrosamine, and yet has adequate strength, elongation and other desired characteristics that it can be used in most applications; as well as an efficient, economical process for manufacturing useful products such as latex gloves from the rubber latex composition.
According to the invention there is provided a thin walled rubber latex material, comprising a thin layer of rubber latex having an oxygen-donating curing agent, an activating agent and at least one of an emulsifier and a stabilizer therein, and wherein the latex is pre-vulcanized as an emulsion prior to being shaped into the layer such that substantially none of the oxygen-donating curing agents remains free in the rubber latex layer. Preferably the oxygen-donating curing agent is a peroxide such as butyl peroxides and alkyl peroxides, or peroxyesters, and combinations thereof, and the rubber latex is natural rubber latex. The rubber latex will be formed using a non-amine anti-oxidizing agent so that it contains substantially no nitrosamine.
According to other preferred aspects of the invention: the rubber latex will additionally include an organic strength enhancing agent such as monomers and/polymers of families of acrylics and esters, together with a sulphur-free cross linking agent such as tert-butyl hydroperoxide or tetraethylenepentamine for cross linking the organic strength enhancing agent to with polysoprene in the natural rubber latex; the thin walled rubber latex material will additionally include a coating over the rubber latex layer for blocking transfer of latex proteins from the rubber latex layer to persons wearing the material and the blocking coating being heat resistant and water resistant such that it remains in a solid, non-flowable state after prolonged exposure to human body temperatures.
Also according to the invention there is provided a process for manufacturing thin walled rubber latex material, comprising the steps of: preparing an emulsion of rubber latex together with an oxygen-donating curing agent, an activating agent and at least one of an emulsifier and a stabilizer, pre-vulcanizing the emulsion until it is stable such that substantially none of the curing agent remains free therein; polymerizing polyisoprene molecules in the rubber latex with an organic strength enhancing agent; forming the stable emulsion into a thin walled material; leaching the formed material to remove undesired latex proteins and chemicals therefrom; and drying the leached, thin walled material. Again, it is preferred that: the oxygen-donating curing agent is preferably a butyl-peroxides, alkyl-peroxides, peroxyesters, and combinations thereof; that the rubber latex is natural rubber latex; that the organic strength enhancing agent comprised polymers and/or monomers of families of acrylics and esters, and that the polymerization step involves the use of an organic cross-linking agent such as tert-butyl hydroperoxide and tetraethylene pentamine; that the process also includes a step of adding nitrogen free anti-oxidant(s) to the latex emulsion such that the manufactured thin walled rubber latex material is substantially free of nitrosoamine, and a step of applying a blocking coating to the thin walled rubber latex material, which coating is substantially impermeable to the latex protein and heat resistant such that it remains in a non-flowable state after prolonged exposure to the human body temperatures.
It is also preferred that the rubber latex material will be formed in the shape of gloves.
It is an object of the invention to provide a thin walled rubber latex material (shaped as gloves or other objects to be worn by humans) which does not contain chemicals or chemical compounds that will react with foreign materials contacted by the thin walled rubber latex material during subsequent use thereof, and to an efficient and economically practical process for manufacturing such rubber latex material.
It is another object of the invention to provide such as thin walled rubber latex material which may be safely and conveniently worn over an extended periods of time by humans without transfer of the latex proteins in the material to human skin in contact therewith.
It is a further object of the invention to provide such a natural rubber latex material having adequate strength, elongation, texture and other characteristics that it may be reliably used for substantially any application in the medical and dental fields.
Other objects, advantages and salient features of the invention will become apparent from the following detailed description which, when taken in conjunction with the indexed drawings, discloses preferred embodiments of the invention.